In wireless communication systems, the task of deciding whether to admit or reject a request from a UE (User Equipment) for a radio bearer setup is handled by a function denoted Admission Control. The Admission Control is typically located in a base station. A connected UE occupies resources for its services, such as e.g. radio transmission resources and licenses. The resources considered by Admission Control are herein referred to as Monitored System Resources (MSRs). For each MSR, it is the aggregated resource usage from existing bearers that is monitored. When a resource is exhausted, or “full”, any further requests to set up a radio bearer, that requires said resource, are rejected by the Admission Control, regardless of the importance of the request.
In order to avoid that important requests are rejected, the concept of Allocation and Retention Priority (ARP) for radio bearers has been introduced by 3GPP (3rd Generation Partnership Project). In LTE (Long Term Evolution), a radio bearer is denoted E-RAB (Evolved-Radio Access Bearer). The ARP includes a “Priority Level”, which is a parameter that describes how important an E-RAB is. Based on the Priority Level, the Admission Control may then prioritize amongst resource requests for setup of E-RABs by means of so-called pre-emption. That is, when one or more resources are exhausted (full) and yet another request for setup of an E-RAB is received, already admitted E-RABs may be pre-empted based on their ARP. Here, “pre-empt” refers to releasing or “shutting down” less important E-RABs.
MSRs may be divided into two main categories; dynamic MSRs and static MSRs. Dynamic MSRs are resources for which the utilization may vary during a bearer's lifetime. These variations may stem from decisions made in a scheduler, as well as from varying radio conditions or mobility.
Static resources, on the other hand, are resources for which the usage does not vary during the lifetime of a bearer. These resources may relate to licenses or hardware/software limitations, or be related to a static model of an otherwise dynamic MSR. The difference in behavior between static and dynamic MSRs is illustrated in FIG. 1. A specific request for setup of a bearer is mapped by Admission Control on to the various MSRs. For a specific request, only some of the MSRs may be relevant. FIG. 4 illustrates how an Admission Control unit 401 receives a request for resources for a bearer set up. The Admission Control unit 401 then sends one request per MSR necessary for setting up the bearer. In FIG. 4, the MSRs are illustrated as to be managed by a number of resource managing entities 402:1-402:n. Each request is answered by a response indicating whether the requested resources may be allocated or not. If one or more responses are negative, the Admission Control rejects the request for bearer setup, and if all responses are positive, the request is admitted.
Some special considerations are needed for dynamic MSRs representing load due to radio bearers with a QoS (Quality of Service) requirement. If too many radio bearers with QoS requirements are admitted, scheduling will at some point fail to provide resources to all of them. Users and bearer setups may have been admitted at a point in time when radio conditions and mobility were favorable in the sense that QoS could be provided. But due to increasing mobility and worsened radio conditions the resources may at a later point in time not be sufficient to provide QoS for the admitted radio bearers. Admission Control strives for having the load due to radio bearers with QoS requirements below a QoS threshold expressed as a percentage of the maximum amount of the resource. It does so by rejecting requests for bearer set up whenever load due to existing bearers with QoS requirements is above the QoS threshold. The QoS threshold could, for instance, relate to the contribution from all the Guaranteed Bitrate Bearers (GBRs). The QoS threshold for a dynamic MSR is illustrated in FIGS. 2 and 3.
The margin created between a QoS threshold and a maximum level, Max Capacity, illustrated in FIGS. 2 and 3, allows for statistical fluctuations with regard to the air interface resources, since the load for high-priority QoS traffic can be limited to a value lower than the maximum level of the resource. Then integrity of the QoS radio bearers is protected with some level of probability, since resources above the threshold are available for the high-prioritized traffic in congested scenarios. Tuning the threshold makes it possible to adjust that probability. Tuning the margin between the threshold and the maximum level of the resource is of special interest when the high-prioritized traffic consists of GBR traffic, where service blocking is desired rather than service dropping. Further, this kind of Admission Control also helps protecting radio bearers without any QoS requirements, since there is a margin allowed for them to use. This means that dynamic MSRs are considered loaded (full) whenever the load exceeds the QoS threshold, whereas static resources can be considered loaded whenever the load equals the maximum level.
As previously described, pre-emption is a method for prioritizing amongst bearers of different priority levels when one or more required resources are loaded. However, pre-emption leads to that users abruptly lose their connection to the network, which is not good from a user experience perspective. Further, pre-emption is not allowed in some markets for certain services. Also, it is difficult to select the most suitable user or bearer to pre-empt. A usual way is to select and pre-empt the bearer that has the worst radio conditions. However, the user associated with this “worst” bearer is probably located close to the cell boarder, and might have be in the process to do a handover when being pre-empted, and would have left the cell anyway.
An alternative to pre-emption is the use of ARP-differentiated thresholds. This means that additional thresholds, one for each ARP, are introduced per MSR, such that radio bearers are admitted based on a comparison of the MSR load with the corresponding threshold. However, the use of such ARP-differentiated thresholds require a very involved configuration, since one threshold for each ARP Priority level needs to be configured per MSR.
A further method for prioritizing amongst bearers of different priority levels is so-called Timer-based blocking. Timer-based blocking means that the Admission Control starts to block radio bearers of certain ARP Priority Levels when there has been a reject from a congested MSR. Initially, all ARP Priority levels are blocked, followed by a timer-based relaxation one-by-one of allowed ARP Priority levels, such that finally all levels are again admitted (if there were no new rejects). However, Timer-based blocking becomes detached from the real resource situation in the MSRs. This is in particular valid for dynamic MSRs with a relation to scheduler-handled resources. It is therefore difficult to design proper settings of the timers.